Shotgun-armed Maryland State police boarded a giant oil tanker in the Chesapeake Bay yesterday and were told that the captain had been threatened and another officer assaulted by angry crew members who shut off the vessel's steam.

No charges were filed and no injuries reported in the alleged incident which reportedly stemmed from a labor dispute about the 38,000 ton Youssef B., anchored yesterday near Annapolis.

However, state police spokesman Bill Clark said last night that "we believe there was a real possibility of a mutiny occurring. All the elements were there."

Police boarded about 2:30 p.m. and left three hours later, but one of the ship's Baltimore agents remained to try to resolve the dispute, which reportedly involved pay. The ship is of Panamanian registery. About 10 of its 30 or so crew members are Polish; the others are of various nationalities.

Clark said state police boarded from motorboats after the ship's captain, Tadeusz Pienkowski, radioed Baltimore asking that pay for crew members be sent immediately.

"The crew otherwise may cause bloody accident," the telex messgae read. It said the vessel was "without steam" which had apparently been "stopped by the crew."

After state and Maryland marine police boarded, Clark said, the captain told them he had fired most or all of the crew yesterday morning for alleged incompetence.

Some of those discharged "were upset" over severance pay and other benefits, Clark said.

According to Clark, the captain claimed that the ship's third engineer had been thrown to the engine room floor by crew members who then shut off the vessel's steam. The ship already was at anchor at the time.

Then, Clark said, crew members allegedly "came after" the captain.

Capt. Pienkowski asked that four crew members be removed from the tanker, Clark said, but when they did not wish to leave, he was told that it would be necessary to go ashore to swear out warrants against them.

Clark said it was not immediately known why the captain had declined to do so.

Coast Guard officials in Baltimore said that the Youssef B.had recently unloaded petroleum products in Baltimore and was carrying only ballast yesterday.